Annual grasses of Mediterranean origin are prevalent invaders in the coastal sage scrub plant community of southern California. This study examined the effects these grasses have on the germination, growth, flowering and survivorship of 0 to 2 year old Artemisia californica Less., a native shrub dominant in the coastal sage scrub. Possible mechanisms leading to these effects were also examined.
To test the direct and indirect effects of grass density on Artemisia, a manipulative field experiment was performed. Grass density was manipulated, and each experimental plot was either planted with two month old Artemisia seedlings, or sown with Artemisia seed. Measurements included soil water potentials and Artemisia xylem water potentials, flower production, biomass, seedling emergence and survival rates. The results of this study indicate that Artemisia germination, first season growth, survivorship and flower production are all negatively related to the density of neighboring annual grasses. Depletion of soil water by the grasses is a likely mechanism behind this reduction of Artemisia growth and survivorship. These results provide information applicable to management and restoration of southern California coastal sage scrub.